In the field of semiconductor technology, research and development efforts are continued for further miniaturization of pattern features. Recently, as advances including miniaturization of circuit patterns, thinning of interconnect patterns and miniaturization of contact hole patterns for connection between cell-constituting layers are in progress to comply with higher integration density of LSIs, there is an increasing demand for the micropatterning technology. Accordingly, in conjunction with the technology for manufacturing photomasks used in the exposure step of the photolithographic microfabrication process, it is desired to have a technique of forming a more fine and accurate circuit pattern or mask pattern.
In general, reduction projection is employed when patterns are formed on semiconductor substrates by photolithography. Thus the size of pattern features formed on a photomask is about 4 times the size of pattern features formed on a semiconductor substrate. In the current photolithography technology, the size of circuit patterns printed is significantly smaller than the wavelength of light used for exposure. Therefore, if a photomask pattern is formed simply by multiplying the size of circuit pattern 4 times, the desired pattern is not transferred to a resist film on a semiconductor substrate due to optical interference and other effects during exposure.
Sometimes, optical interference and other effects during exposure are mitigated by forming the pattern on a photomask to a more complex shape than the actual circuit pattern. Such a complex pattern shape may be designed, for example, by incorporating optical proximity correction (OPC) into the actual circuit pattern. Also, attempts are made to apply the resolution enhancement technology (RET) such as modified illumination, immersion lithography or double exposure (or double patterning) lithography, to meet the demand for miniaturization and higher accuracy of patterns.
The phase shift method is used as one of the RET. The phase shift method is by forming a pattern of film capable of phase reversal of approximately 180 degrees on a photomask, such that contrast may be improved by utilizing optical interference. One of the photomasks adapted for the phase shift method is a halftone phase shift mask. Typically, the halftone phase shift mask includes a substrate of quartz or similar material which is transparent to exposure light, and a mask pattern of halftone phase shift film formed on the substrate, capable of providing a phase shift of approximately 180 degrees and having an insufficient transmittance to contribute to pattern formation. As the halftone phase shift mask, Patent Document 1 (JP-A H07-140635) proposes a mask having a halftone phase shift film of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON).
For the purpose of forming finer images by photolithography, light of shorter wavelength is used as the light source. In the currently most advanced stage of lithography process, the exposure light source has made a transition from KrF excimer laser (248 nm) to ArF excimer laser (193 nm). The lithography using ArF excimer laser light of greater energy was found to cause damages to the mask, which were not observed with KrF excimer laser light. One problem is that on continuous use of the photomask, foreign matter-like growth defects form on the photomask. These growth defects are also known as “haze”. The source of haze formation was formerly believed to reside in the growth of ammonium sulfate crystals on the mask pattern surface. It is currently believed that organic matter participates in haze formation as well.
Some approaches are known to overcome the haze problem. With respect to the growth defects formed on the photomask upon long-term irradiation of ArF excimer laser light, for example, Patent Document 2 (JP-A 2008-276002) describes that if the photomask is cleaned at a predetermined stage, then it can be continuously used.
As the exposure dose of ArF excimer laser light irradiated for pattern transfer increases, the photomask is given damage different from haze; and the size of the mask pattern changes in accordance with the cumulative irradiation energy dose, as reported in Non-Patent Document 1 (Thomas Faure et al., “Characterization of binary and attenuated phase shift mask blanks for 32 nm mask fabrication,” Proc. of SPIE, vol. 7122, pp 712209-1 to 712209-12). This problem is that as the cumulative irradiation energy dose increases during long-term irradiation of ArF excimer laser light, a layer of a substance which is considered to be an oxide of the pattern material grows outside the film pattern, whereby the pattern width changes. It is also reported that the mask once damaged cannot be restored by cleaning with AMP (aqueous ammonia/hydrogen peroxide) as used in the above-mentioned haze removal or with SPM (sulfuric acid/hydrogen peroxide). It is believed that the damage source is utterly different.
Non-Patent Document 1 points out that upon exposure of a circuit pattern through a halftone phase shift mask which is the mask technology useful in expanding the depth of focus, substantial degradation is induced by pattern size variation resulting from alteration of a transition metal/silicon base material film such as MoSi base material film by irradiation of ArF excimer laser light (this degradation is referred to as “pattern size variation degradation”). Then, in order to use an expensive photomask over a long period of time, it is necessary to address the pattern size variation degradation by irradiation of ArF excimer laser light.
As pointed out in Non-Patent Document 1, the pattern size variation degradation by irradiation of short wavelength light, typically ArF excimer laser light does scarcely occur when light is irradiated in a dry air atmosphere. Exposure in a dry air atmosphere is regarded as a new approach for inhibiting the pattern size variation degradation. However, the control of a dry air atmosphere adds an extra unit to the exposure system and gives rise to electrostatic and other problems to be managed, leading to an increased expense. Under the circumstances, it is necessary to enable long-term exposure in a common atmosphere that does not need complete removal of humidity (typically having a humidity of around 50%).
The photomasks used in the lithography using ArF excimer laser light as light source include halftone phase shift masks having a halftone phase shift film of a silicon base material containing a transition metal, typically molybdenum. This silicon base material is mainly composed of a transition metal and silicon, and further contains oxygen and/or nitrogen as light element (e.g., Patent Document 1). Suitable transition metals used include Mo, Zr, Ta, W, and Ti. Among others, Mo is most often used (e.g., Patent Document 1). Sometimes a second transition metal is added (e.g., Patent Document 3). For the light-shielding film, silicon base materials containing a transition metal, typically molybdenum are also used. However, when a photomask using such transition metal-containing silicon base material is exposed to a large dose of high-energy radiation, the mask undergoes significant pattern size variation degradation by irradiation of high-energy radiation. Then the service lifetime of the photomask is shorter than the requirement.
It is a serious problem that when a photomask pattern on a halftone phase shift mask is irradiated with short-wavelength light, typically ArF excimer laser light, the photomask pattern for exposure experiences a variation of line width, that is, “pattern size variation degradation.” The permissible threshold of pattern width differs with the type of photomask pattern, especially the pattern rule applied thereto. If variations are small, the mask may be further used by correcting the exposure conditions and resetting the irradiation conditions of an exposure system. For example, in the lithography for forming semiconductor circuits complying with the pattern rule of 22 nm, the variation of mask pattern line width must fall within approximately ±5 nm. However, if a pattern width variation is large, there is a possibility that the variation has an in-plane distribution on the photomask. Also in the further miniaturization technology, an auxiliary pattern having an ultrafine size of less than 100 nm is formed on the mask. For the purpose of pattern miniaturization on these masks and from the aspect of an increase of mask processing cost by complication of mask pattern, there is a need for a halftone phase shift mask film which experiences minimal pattern size variation degradation and allows for repeated exposure.